The invention relates to a colour television coding circuit suitable for use in a colour television system in accordance with the NTSC standard, the coding circuit comprising a series arrangement formed by a first matrix circuit for forming colour difference signals, a second matrix circuit for forming standardized I- and Q-signals therefrom, first and second low-pass filters following the second matrix circuit and having different frequency passbands for the I- and the Q-signal, respectively, a modulator circuit following the filters for quadrature modulation of a chrominance subcarrier by the frequency-limited I- and Q-signals and a first superposition circuit following the modulator circuit for forming a chrominance signal from the modulated signal, the coding circuit further comprising a second superposition circuit for inserting a burst-gate signal into the series arrangement for obtaining in a chrominance signal occurring at a coding circuit output a burst of the chrominance subcarrier during a standardized period of time.
Such a coding circuit is described in the book "Television Engineering Handbook", by D. G. Fink, first edition 1957, pages 9-42 to 9-45, inclusive. In the described implementations of the coding circuit, the burst-gate signal of a fixed duration for obtaining the standardized chrominance subcarrier burst in the chrominance signal is used during signal processing in the series arrangement after the low-pass filters. Should this not be done, and the burst-gate signal were inserted before the filters in the series arrangement, the result would be that the steepness of the pulse edges in the burst-gate signal is smoothed to an impermissible extent. Seeing that the pulse edge steepness' attenuation by the I-signal filter having a frequency passband of 0 to 1.2 MHz is already impermissible, then this is certainly the case for the Q-signal filter having a frequency passband of 0 to 0.5 MHz. An advantage of inserting the burst-gate signal before the low-pass filters in the series arrangement is the presence of the signal before the said second matrix circuit for forming the I- and Q-signals. The burst-gate signal might then be inserted into one of the colour difference signals, more specifically in anti-phase in the colour difference signal (B-Y), as a result of which the burst-gate signal thus inserted in the proper phase may be matrixed in the I- and Q-matrix circuit, and the burst-gate signal components might here and in the modulator circuit maintain the proper phase relationship and ultimately result in a chrominance subcarrier burst in the chrominance signal with the proper phase and amplitude.